Polyolefins typified by polyethylene, as well as ethylene copolymers and their crosslinked products, generally exhibit excellent electrical insulating properties and have been extensively used as insulating materials for power cables and other electrical equipment.
However, if these materials are used either under wet conditions or in the water, their high insulating performance will be greatly impaired because of the growth of bow-tie trees in the insulations. In the worst case, the bow-tie trees might pass through the insulation layer to cause an accident of dielectric breakdown. The bow-tie trees can be explained by the phenomena that water molecules in a gaseous phase that permeate the insulation upon current application diffuse into fine imperfections such as foreign matters and microvoids; as they condense, the water molecules will grow enough to distort the polymer chains and eventually the water displaces the latter and emerges in the form of bow-tie tree from the insulation.
With the state of the art, the occurrence of imperfections in the insulation such as foreign matters and microvoids can be prevented to some extent by controlling the quality of resins or improving the cable fabrication technology but it is still impossible to eliminate all such imperfections. So long as foreign matters and microvoids which provide nuclei for bow-tie trees remain, it is essential to prevent excessive concentration of the water in imperfections. To this end, water must be uniformly distributed in the insulation so as to render the resin appropriately hydrophilic.
Based on this observation, it has been proposed that an aliphatic acid containing a hydroxyl group in the molecule or a metal salt thereof can be added to a polyolefin or an ethylene copolymer, to significantly improve the resistance to bow-tie trees of the polymer (Japanese Patent Laid-Open Application No. 220304/1983).
However, even this method is incapable of complete prevention of the occurrence of bow-tie trees. In order to enhance the long-term reliability of crosslinked polyethylene insulated cables for high voltages and various other electrical equipment, further improvements in their resistance to bow-tie trees are desired.